The present invention relates to a color cathode ray tube and more specifically to a color cathode ray tube which has an improved purity (color purity) adjustment margin and increased brightness, without degrading resolution of display images.
A cathode ray tube of this kind includes at least a vacuum vessel consisting of a face panel, a funnel and a neck, all integrally connected together; a phosphor surface formed by applying a phosphor to the inner surface of the face panel; a shadow mask installed inside the face panel and suspended close to the phosphor surface; and an electron gun installed inside the neck. The electron gun emits, for example, three electron beams, which are color-selected by the shadow:mask and strike the phosphor to reproduce a desired picture.
The phosphors of generally three primary colors, embedded in a specific order in black matrix holes of various shapes such as dot, stripe or rectangle, are applied to the inner side of the face panel to form the phosphor surface.
The shadow mask consists of a metal plate formed with a large number of electron beam passage apertures, each of which has a color selection function to make the electron beam hit the phosphor of an appropriate primary color on the phosphor surface.
The black matrix holes in which to embed the phosphors are generally in the form of a stripe or rectangle for so-called television color cathode ray tubes, whereas color cathode ray tubes, such as display monitors, that require precise and detailed image display, employ dot-shaped black matrix holes or those of similar shapes.
The electron beam passage apertures in the shadow mask installed inside the face panel generally have a similar shape to the phosphors. The cathode ray tube with dot-shaped phosphors (dot-shaped black matrix holes) uses a shadow mask formed with circular electron beam passage openings.
FIG. 4 is a schematic diagram showing a black matrix pattern on the phosphor surface of a conventional color cathode ray tube that has dot-shaped phosphors and circular electron beam passage apertures. Reference numeral 40g represents a black matrix hole in which to embed a green phosphor G, 40b a black matrix hole in which to install a blue phosphor B, and 40r a black matrix hole in which to place a red phosphor R.
As shown in the figure, the black matrix pattern formed on the conventional phosphor surface is generally arranged in such a way that the lines connecting the centers of the black matrix holes of the same color form a regular triangle, with the center-to-center distances between the adjacent colors (pitches) p being equal.
Therefore, the distance between the black matrix holes of the same color in a vertical direction V in which the beam is deflected at a relatively low deflection frequency is p, whereas the distance between the black matrix holes of the same color in a horizontal direction H in which the beam is deflected at a relatively high frequency is .sqroot.3.multidot.p.
As an example of conventional technique concerning the cathode ray tube of this kind, Japanese Patent Laid-Open No. 100338/1983 may be cited.
There is known a conventional art which has an increased distance (pitch) between the phosphors of the same color in the vertical direction V to enhance the landing margin of the electron beam for the terrestrial magnetism.
FIG. 5 is a schematic diagram showing a conventional phosphor surface with the vertical pitch of the black matrix pattern extended. Parts identical with those of FIG. 4 are assigned like reference numerals.
In the phosphor surface of FIG. 5, the distance b in the vertical direction V between lines connecting in the horizontal direction H the centers of the black matrix holes is set 50% greater than the center-to-center distance a between the horizontally adjacent black matrix holes, that is, b/a=1.50.
Thus, if the distance (pitch) between the adjacent black matrix holes (phosphors) of the same color is taken to be p, the distance between the vertically adjacent black matrix holes of the same color is .sqroot.2.multidot.p. Also, the distance between the horizontally adjacent black matrix holes (phosphors) of the same color is also .sqroot.2.multidot.p.
Literature that disclose the prior art cathode ray tube of this kind include Japanese Patent Laid-Open No. 25657/1982.
The above-mentioned color cathode ray tube having dot-type phosphors has many advantages over the so-called slit matrix system (Trinitron type color cathode ray tube (trade name)) in terms of adjustability of superimposition of electron beams of individual primary colors, such as enhanced convergence, which is realized by the use of a face panel having a larger spherical curvature. For luminance and purity, however, the slit matrix system is advantageous.
In the phosphor surface shown in FIG. 4, the horizontal pitch between the black matrix holes of the same color is .sqroot.3 times the vertical pitch.
FIG. 6 is a diagram explaining the transmission factors of the phosphor surface of the conventional color cathode ray tube shown in FIG. 4. As shown in the figure, if the vertical distance (pitch) between the phosphors of the same color is taken to be p (=210 .mu.m) (see FIG. 4), then the vertical distance between the vertically adjacent phosphors p/2 (=105 .mu.m), and the distance between the horizontally adjacent phosphors is p/.sqroot.3 (=210/.sqroot.3=120 .mu.m).
Then if the interval (guard band) between the horizontally adjacent phosphors C1 is 40 .mu.m, and the diameter of the black matrix hole (diameter of the phosphors) C2 is 80 .mu.m, then the transmission factor is (.pi./4.times.80.sup.2)/(120.times.105).times.100=39.9%.
The resolution Of the phosphor surface is determined by the resolution in the horizontal direction in which the pitch is wider. The smaller pitch in the vertical direction reduces the purity margin, lowering the brightness.
Therefore, when phosphor surfaces of dot type and of stripe type with the same resolution are compared, the transmission factor of the black matrix is lower for the dot type phosphors than for the stripe type phosphors. In other words, the dot-type phosphors have a problem of reduced brightness.